1. Field of the Invention
The present invention relates to a method which adjusts the concentration of oxygen in a silicon (Si) single crystal and which is used in a single crystal pulling apparatus using the Czochralski method and to a apparatus for use in the adjusting method.
2. Description of Prior Art
When a Si single crystal is pulled up from Si molten liquid in a quartz crucible using the CZ method, volatile silicon oxide (SiO) is produced by the reaction between the quartz crucible and the Si molten liquid and deposits on an edge of the crucible, the Si single crystal, a pulling shaft and an inside wall of a chamber. The SiO which deposits on the pulling shaft which is ascending while rotating is scraped by an airtight seal ring and falls into the molten liquid below, so that the single crystal being produced is damaged. In order to solve this problem, a method is suggested (in Japanese Examined Patent No. 54-6511) in which a purge tube is hung down to be concentric with the pulling shaft in the chamber, argon (Ar) gas is allowed to flow down in a straightening tube from above, and the SiO which evaporates from the surface of the molten liquid together with the Ar gas is exhausted through the bottom of the chamber.
On the other hand, the SiO is also absorbed into the Si single crystal. Since oxygen in the Si single crystal functions as the main element of gettering effect against impurities intruding in the process of producing an integrated circuit, it has been conventionally thought that a preferable concentration of oxygen in the Si single crystal should be approximately 20 ppma (by ASTM F121-83). However, the integration of a semiconductor device has recently reached a high level, and it has been revealed that oxygen in the Si single crystal causes an oxygen inducing fault (OISF) and a swirl fault. As a result, manufacturers of semiconductor devices need a Si single crystal in which the concentration of oxygen is lower than 15 ppma.
The following methods are provided in order to lower the concentration of oxygen in the Si single crystal:
(A) to reduce the reaction between the Si molten liquid and the quartz crucible by decreasing the number of rotations of the quartz crucible PA1 (B) to increase the amount of volatilized SiO by increasing the flow rate of inert gas flowing through the straightening tube PA1 (C) to increase the amount of volatilized SiO by lowering the pressure in the chamber
However, in the case of the method (A), if the number of rotations is too small, the temperature distribution in the Si molten liquid is prevented from being even. In the method (B), since the flow rate of the inert gas is required to be considerably high (normally, 50-100 l/min increased), its costs are increased. In the case of the method (C), if the pressure is too low, the reaction between the quartz crucible and the Si molten liquid becomes large, the life of the quartz crucible is shortened, and the production costs are increased.